Apple will be resetting iCloud Backup data on Thursday, September 22. It is believed that the reset is being performed for the release of iOS 5 GM, which is rumored to be released around the same time. iOS 5 GM will be the last release before the oficial debut in early October.

From Apple Developer Board:

On Thursday, September 22, the iCloud Backup data will be reset. Backing up to iCloud or restoring from an iCloud backup will be unavailable from 9 AM PDT – 5 PM PDT. If you attempt a backup or restore during this time, you will receive an alert that the backup or restore was not successful. After this reset, you will be unable to restore from any backup created prior to September 22. A full backup will happen automatically the next time your device backs up to iCloud.